Increasing environmental restrictions and regulations are causing diesel engine manufacturers and packagers to develop technologies that improve and reduce the impact that operation of such engines have on the environment. As a result, much design work has gone into the controls that operate the combustion process within the engine itself in an attempt to increase fuel economy and reduce emissions such as NOx and particulates. However, given the operating variables and parameters over which a diesel engine operates and given the tradeoff between NOx and particulate generation, many engine manufacturers and packagers have found it useful or necessary to apply exhaust aftertreatment devices to their systems. These aftertreatment devices are used to filter or catalytically refine the exhaust gas flow from the diesel engine to remove or reduce to acceptable levels certain engine exhaust emissions and typically have specific thermal operational requirements in order to function effectively.
One such exhaust aftertreatment device is called a Diesel Particulate Filter (DPF). The DPF is positioned in the engine exhaust system such that all exhaust gases from the diesel engine flow through it. The DPF is configured so that the soot particles in the exhaust gas are deposited in the filter substrate of the DPF. In this way, the soot particulates are filtered out of the exhaust gas so that the engine or engine system can meet or exceed the environmental regulations that apply thereto.
While such devices provide a significant environmental benefit, as with any filter, problems may occur as the DPF continues to accumulate these particulates. After a period of time, the DPF filter substrate becomes sufficiently loaded with soot causing the exhaust gases to experience a significant pressure drop passing through the increasingly restrictive DPF. As a result of operating with an overly restrictive DPF, the engine thermal efficiency declines due to the fact that the engine must work harder and harder simply to pump the exhaust gases through the loaded DPF. This loss of engine performance, due to increased restriction in the exhaust system, continues to grow more severe with continued engine operation and DPF soot accumulation, eventually culminating in engine failure or engine shutdown.
To avoid such an occurrence, the engine packager typically incorporates one of several possible filter heating devices upstream of the DPF to periodically clean the filter of the accumulated soot. These filter heating devices are used periodically to artificially raise the temperature of the exhaust stream entering the DPF to a point at which the accumulated soot will oxidize and burn using the residual oxygen in the exhaust stream. When initiated at a time before loading of the DPF becomes excessive, the ignition and burn off of the trapped particulate matter will occur in a safe and controlled fashion. This process of burning the soot from the DPF filter substrate in such a controlled manner is called regeneration. One important parameter of a DPF regeneration is the temperature uniformity going into the DPF during the regeneration event. Localized areas of the DPF that are warmer or cooler than the targeted regeneration temperature can decrease the effectiveness of the regeneration event. Warmer areas may cause filter damage through thermal gradients or accelerated soot oxidation while cooler areas may result in areas of the filter that are not cleaned of soot.
Other engine exhaust aftertreatment devices include Diesel Oxidation Catalysts (DOC), Urea-Selective Catalytic Reduction systems (SCR), Lean-NOx traps, and many others. Many of these devices rely on catalytic reactions occurring with chemicals or exhaust emissions on a substrate surface. Some of these devices incorporate injections of chemicals into the exhaust stream upstream of the substrate. Temperature and chemical uniformity of the flows entering these devices is critical to efficient operation of these devices. Use of the expensive catalysts on the substrate surface is maximized when the entire substrate experiences uniform temperature and chemical mixtures within the desire range. Areas of the substrate that experience flows outside of the target temperature or chemical composition will degrade performance of the aftertreatment system.
Typically a DOC must operate above temperatures of approximately 350 C and an SCR system must operate above temperatures of approximately 300 C. Operation of either of these systems in temperatures lower than specified results in decreased performance and efficiency of the system. Lean-NOx traps also have temperature limitations but further have the requirement of specific oxygen concentrations at periodic intervals. In order for a Lean-NOx trap to operate and purge its catalytic surface of accumulated NOx, exhaust with low oxygen and high hydrocarbon content must be passed through the substrate. This is an added requirement of the system.
As previously discussed many diesel exhaust aftertreatment devices have characteristic operation temperature and sometimes species composition requirements. A difficulty with these systems is that the engine exhaust may not be of sufficient temperature or composition at all times to maintain operation of these devices. Many methods have been devised to provide the auxiliary heat or species concentrations necessary for proper aftertreatment device operation. For example, the operating parameters of the diesel engine may be modified in such a manner to cause the exhaust temperature to rise to a level sufficient for proper operation of the devices. It is also possible to inject hydrocarbon fuel into the exhaust of a diesel engine immediately before the exhaust passes through a Diesel Oxidation Catalyst (DOC). The DOC converts the excess hydrocarbon fuel in the exhaust stream into heat by means of the catalytic reaction of exhaust oxygen with hydrocarbons on the catalyst, thus increasing the exhaust gas temperature prior to its passage through other aftertreatment devices. However, as previously mentioned, the DOC has its own temperature limitations and heat addition may be required prior to the DOC to insure proper operation. Supplemental heat may also be generated in the exhaust flow by use of an auxiliary electrical heater placed within the exhaust path. This supplemental heat is added to the exhaust gas prior to its passage through the aftertreatment devices. As an alternative to the use of an electric heater, another method of filter regeneration uses a fuel-fired burner or combustor to heat the exhaust gas.
The challenge when using a fuel-fired burner to perform this heat addition is to create a combustor that will raise the temperature of exhaust gasses while meeting criteria for light-off performance, combustion stability, emissions, and exhaust pressure loss. Additionally, it may be necessary to operate the fuel-fired burner in a mode that reduces the remaining engine exhaust oxygen content while supplying excess hydrocarbons subject to the above requirements. Both of these combustor operational modes have challenges when operating in the diesel exhaust environment. Specifically, the operational conditions in the diesel engine exhaust system differ severely from other operating environments where combustors are used, e.g. in gas turbine engines. As such, the combustor must operate over a wide range of exhaust flow rates, temperatures, and oxygen concentrations. In the diesel exhaust stream, oxygen concentrations can range from approximately 3 to 19% by mass and other diluents such as CO2 and H2O may be present in large quantities. As the diesel engine is operated through various conditional states, the composition and temperature of the engine exhaust which is use as the oxidizer in the combustor can change greatly in magnitude as well as in a very short time. These changes in the oxidizer supplied to the exhaust system combustor create a difficult environment for stable, sustained combustion that meets the above described requirements.
As a result of the difficult combustion requirements demanded of a combustor that operates in an engine exhaust system, new designs and innovations in the area of combustor systems are required. It has been established that combustors that utilize multiple stages for the combustion process can be beneficial for this application. U.S. Pat. No. 4,951,464 by Eickhoff et al., teaches that a fuel rich mixture can be partially combusted in an isolated “Primary” combustion chamber with the remaining fuel being later oxidized in a subsequent combustion chamber with the addition of engine exhaust gases to provide oxygen to complete the combustion of the remaining fuel. However, the combustor design as described in Eickhoff is deficient due to the design's inability to control the amount of engine exhaust that is introduced to combust the remaining fuel. Addition of too little engine exhaust to complete combustion of the fuel will result in an incomplete reaction resulting in elevated hydrocarbon emissions from the device. Addition of too much engine exhaust to combustion reaction for the residual fuel will cause combustion quenching, again resulting in elevated hydrocarbon emissions.
A further typical requirement of some aftertreatment devices is for uniform temperature distribution entering the device. Typical average temperature requirements of aftertreatment devices are also typically much lower than the burner combustion temperatures. Because of the need to lower the combustion temperatures to acceptable levels, all or a portion of the engine exhaust is often diverted from the combustion process and used to cool the products of the combustion event. This recombination of hot and relatively cool gasses presents significant challenges when trying to meet temperature uniformity requirements for the aftertreatment devices located downstream of the combustor. Typical combustor designs have a combustion region near a center axis of the combustor assembly and divert engine exhaust into an annular passage around the combustor resulting in an output temperature profile that is characteristically hot in the center and cooler towards the outside. U.S. Pat. No. 4,651,524 by Brighton teaches such a typical combustor assembly. Additionally, a flame arrestor device such as that disclosed by Brighton may excessively restrict the flows of the combustor device and have stringent material requirements due the high temperatures typical of the operation of such a device.
U.S. Pat. No. 4,538,413 by Shinzawa et al., and U.S. Pat. No. 4,541,239 by Tokura, et al., both teach the concept of diverting all or a portion of the engine exhaust around the combustion event. The bypassed engine exhaust products then must pass through holes or openings in a combustion liner or other device to be mixed with the products of the combustion before exiting the combustor. This approach is disadvantageous due to inherent requirement of the mixing mechanism on pressure drop across the openings or holes. This type of mixing device requires jets of the cooler exhaust to be formed to rapidly mix the exhaust with the combustion product. The mixing effectiveness is dependent on the kinetic energy of the jets which is a direct result of the pressure drop across the openings where the jets are formed. A complication of the application of such devices to engines is that the engines typically have a very wide exhaust flow range. The wide range of flows through the combustor device result in a wide range of flows across these openings and thus a wide range of pressure drops across the openings. Therefore, jet style mixers typically have inefficiencies that cannot be fully mitigated. Low flows do not produce sufficient pressure loss across the mixer to provide high energy jets, resulting in poor mixing. High flows produce excessive pressure loss across the mixer at high flows resulting in system inefficiencies and increased engine fuel consumption due to excessive backpressure on the engine.